This invention relates to a surveying method and is particularly, though not exclusively, applicable to the surveying of the surface of an existing roadway in order to obtain measurements of level at desired points on the roadway.
Measurements of level are required at desired points along the line of an existing roadway, which may be subject to undue wear or subsidence following heavy usage, to facilitate calculations being made as to the quantities of surfacing operations. Accurate indications of level will also be required, as reference heights, during the application of subsequent layers of paving material or the like during re-surfacing.
At one time, levelling measurements were carried out by a team of a surveyor operating a surveyor's levelling instrument, and a "chain boy" carrying a measuring staff. By first deriving the level of the instrument from a reference mark, the level of the foot of the staff at any required measurement point can readily be calculated by reading-off the height of the staff as viewed horizontally through the instrument, and then subtracting this reading from the previously derived height of the instrument.
More recently, road surfaces have been measured by observing angles and distances to a retroreflective target. The road surface is normally measured from predetermined co-ordinate positions over a range of 0 to 100 meters. To establish these co-ordinated points, two processes are involved: (a) traversing to establish x-y co-ordinates and (b) levelling to establish z co-ordinates. These points are established typically at 150 meter intervals along the route, so that the entire road surface is within range of at least one point.
However, even with these more modern methods, it is necessary to close-off sections of a roadway e.g. one or more lanes of a motorway or freeway, so that the measurement can be carried out by the operating personnel without risk of being struck by high speed traffic. This involves a high cost of coning-off freeways, with necessary labor costs involved, as well as disruption to the pattern of traffic flow.
Laser-type measuring instruments, have recently been developed which employ low power infra red beams, and permit distance measurements to passive targets such as the road surface itself. However, there is a range restriction on the use of this method on horizontal surfaces to required levels of accuracy, though acceptable results could be obtained in the range 10 to 30 meters from an elevated instrument, using specially developed techniques.
U.K. Patent Specification 2123941A discloses a surveying method which uses a stationary laser light source together with two measuring instruments such as theodolites. The method comprises projecting light from the stationary light source to a point on the road surface; the two theodolites are then aligned with the point and the theodolites readings are taken.
The invention is therefore based on the use of these new style instruments, and enables a surface of a road to be surveyed without retroreflective targets, and from a position at the side of the road which should give rise to large cost savings, and avoidance of the confusion and chaos which normally results from lane closures required by conventional techniques.